Sintered hard metal product

ABSTRACT

AN IMPROVED SINTERED HARD METAL COMPOSITION IS PROVIDED COMPRISED ESSENTIALLY OF REFRACTORY METAL COMPOUND GRAINS OR PARTICLES COATED WITH A REFRACTORY CARBIDE LAYER TO IMPART WETTABILITY TO REFRACTORY METAL COMPOUND GRAINS WHICH ARE DIFFICULT TO WET WITH SUCH BINDER METALS AS THE IRON GROUP METALS. THE WETTABLE REFRACTORY CARBIDE COATING IS FORMED BY DEPOSITING A LAYER OF THE CORRESPONDING REFRACTORY METAL UPON THE GRAINS FROM A HALIDE OF THE METAL, WHICH LAYER IS THEREAFTER CARBURIZED; OR THE CARBIDE COATING MAY BE PRODUCED SIMULTANEOUSLY ON THE SURFACE OF THE GRAINS FROM AN ATMOSPHERE COMPRISING HALIDE VAPOR IN THE PRESENCE OF A CARBURIZING AGENT, IN WHICH HYDROGEN MAY OR MAY NOT BE PRESENT. THE COATED REFRACTORY METAL COMPOUND GRAINS MAY OPTIONALLY BE MIXED WITH WETTABLE REFRACTORY METAL CARBIDE GRAINS IS PRODUCING SINTERED HARD METAL COMPOSITIONS.

United, States Patent US. Cl. 29-1825 31 Claims ABSTRACT OF THEDISCLOSURE 'An improved sintered hard metal composition is providedcomprised essentially of refractory metal compound grains or particlescoated with a refractory carbide layer to impart wettability torefractory metal compound grains which are difiicult to wet with suchbinder metals as the iron group metals. The wettable refractory carbidecoating is formed by depositing a layer of the corresponding refractorymetal upon the grains from a halide of the metal, which layer isthereafter carburized; or the carbide coating may be producedsimultaneously on the surface of the grains from an atmospherecomprising halide vapor in the presence of a carburizing agent, in whichhydrogen may or may not be present. The coated refractory metal compoundgrains may optionally be mixed with wettable refractory metal carbidegrains in producing sintered hard metal compositions.

This application is a continuation of application Ser. No. 7,970, nowabandoned, filed Feb. 2, 1970.

This invention relates to improved sintered hard metal products, such ascemented refractory metal carbides, and also to a method for producingsuch sintered products from those refractory metal compound grains notreadily wetted by iron group type binder metals.

Recent developments in new alloy materials, such as new and improvedhigh temperature alloys for use as structural elements in heat enginecomponents of jet aircraft and in other structures calling for highstrength and corrosion resistance at elevated temperatures, have placedheavy demands for new and improved hard metal products, e.g. cementedcarbides, capable of machining such alloys.

' The trend has been towards producing harder sintered hard metalshaving improved wear resistance; however, many of the hard metalcompounds which show any promise are not readily wetted by such bindermetals as the iron group metals iron, nickel and cobalt. Thus, due topoor sintering, the elasticity or ductility of the binder metal is notfully utilized in' the final product. The term sinteredhard metalemployed herein relates to those p'r'oductsmade by sintering togetheracompressed powder mixture of refractory metal compounds, such asrefractory metal carbides, borides and nitrides, and a ductible bindermatrix metal, such as iron, nickel, cobalt, and the like.

It has long been known that several very hard refractory metalcompounds, such as titanium carbide, titanium boride, tungsten boride,and the like, are not readily wettable and, therefore, are difficult tobond to iron group binder metals, such that the sintered productgenerally tends to be quite brittle and of little industrial interest.These refractory compounds, on the other hand,.are at- 3,752,655Patented Aug. 14, 1973 ice tractive in view of their low initial cost.In attempts to utilize such hard refractory metal compounds, effortshave been made to improve their wettability relative to the binder metalby adding other metals to their binder metal prior to sintering.

Under certain circumstances, the added metal can form a mixed carbidelayer on the solid hard materials during sintering. However, this methodresults in a rather dilfused surface layer on the hard material, whereinthe bonding property is not appreciably improved. One system which hasbeen proposed (note US. Pat. Re. 25,815) is the system TiC-Ni (Mo),where molybdenum or molybdenum carbides are added to the nickel bindermetal before sintering and where the TiC-powder is substantially free ofmolybdenum. It is further known according to Swedish Pat. 314,212 thatmolybdenum metal can be added to a hard metal system containg TiC insuch manner that the Mo-metal is deposited on the TiC-cores beforesintering to provide improved dispersion of M0 in the system. However,this method results only in a marginal improvement with respect to thefirst-mentioned method. With regard to the known techniques for addingmetal to the binder phase, it is important to note that when the powderof hard material consists of borides and nitrides, no carbides areformed on the surface of the particles as discussed above, but insteadmixed borides and nitrides, respectively, are formed, which generallymeans poor wetting properties and a brittle sintered hard metal body.

It has been found in accordance with the invention that theaforementioned difiiculties can be overcome by directly modifying thesurface of the refractory metal compound grains prior to sintering sothat they are rendered more wettable and can be mode easily sintered ina matrix of an elastic or ductile binder metal phase consistingsubstantially of one or more of the iron group metals. The sintered hardmetal obtained is generally very hard, has a high water resistance andgood modulus of elasticity.

It is thus an object of this invention to provide an irn: provedsintered hard metal composition consisting essentially of refractorymetal compound grains not readily wettable in which the surface of thegrains has been modilied with a special layer of refractory metalcarbide in order to confer improved wettability to the refractorycompound grains relative to the binder metal inwhich the grains aredispersed.

Another object is to provide a method of producing an improved sinteredhard metal composition from refractory metal compound grains normallydifficult to wet with binder metal in which the surface of the grains isfirst modified by applying a special refractory carbide coating prior tosintering to enhance the wettability of the refractory compound grainsrelative to the binder metal, such as iron group binder metals.

These and other objects will more clearly appear when taken inconjunction with the following description and the appended claims.

Stating it broadly, the invention provides as a product an improvedsintered hard metal composition consisting essentially of refractorycarbide-coated hightemperature refractory metal compound grainsdispersed through a binder matrix metal selected from the groupconsisting of Fe, Ni, Co and Fe-base, Ni-base and Co-base alloys, thecoated refractory metal compound grains having a core selected from thegroup consistingvof: 1) monocarbides of the Group IVb and Group Vbelements Ti,

Zr, Hf, V, Nb and Ta, and (2) nitrides and borides of the Group IVb,Group Vb and VIb elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W, therefractory carbide coating on each of said cores being selected from thegroup consisting of carbides of V, Nb, Ta, Cr, Mo and W, the carbides ofV, Nb and Ta having the formula selected from the group consisting of MeC and MeC the compound MeC having a cubic structure in which x is lessthan 1, and more preferably ranging from about 0.75 to about 0.85.

A method which may be employed for producing coated fine granular corematerial comprising refractory metal compound grains comprisesintimately contacting a powder of the core material of desired particlesize with a fluid of one or several halides of the metals selected fromthe group V, Nb, Ta, Cr, Mo and W, preferably an atmosphere of hydrogenwhich may also contain a hydrocarbon gas. For example, by having ahydrocarbon gas present, the coating deposited directly on the corematerial may be a carbide of one or more of the elements V, Nb, Ta, Cr,M or W. Alternatively, the method may be carried out in two steps, towit: substantially pure metal may be deposited on the core material fromthe halide vapor of the metal at an elevated temperature in preferablyan atmosphere of hydrogen; and in the second step, the deposited metalmay then be carburized using a hydrocarbon gas.

As stated above, it is known from the literature that a carbide of thetype MeC, in which Me is a metal from Group IVb and Vb in the periodicsystem, i.e., TiC, ZrC, HfC, VC, NbC and TaC, is not readily wetted byliquid metals from the iron group, i.e. Ni, Fe and Co. The wetting ofthe carbides MeC and the bonding to the iron metals can, however, beconsiderably improved if the carbon content is reduced in thehomogeneity range of the crystal lattice. Carbides of the type Me C,wherein Me is a metal from the Group Vb, are also more easily wettedthan the indicated carbides of the type MeC. The carbides of the metalsin Group VIb, e.g. Cr C M0 0 and WC, are readily wetted by the metals ofthe iron group. Generally speaking, refractory metal carbides areusually more readily wetted than, for instance, refractory metalnitrides.

It has been found that hardness as a function of the carbon content ofcarbides of metals in the Groups Nb and Vb behave rather strangely. Theintrinsic hardness is greatly reduced with decreasing carbon content forcarbides in Group IVb, i.e. for TiC, ZrC and HfC and for VC; whereas,the intrinsic hardness increases with decreasing carbon content for NbCand TaC. The variations of the hardness with the carbon content arequite significant for all the carbides. In the following table, thehardnesses are shown measured as Vickers hardness using a 50 g. weight:

I-I (Vickers hard- Refractory carbide: ness), kg/mm. TaC TaC 3000 NbC1400 NbC 3200 maa 3000 C 2000 TiC 3000 TiC 1800 According to the presentinvention, a core or grain of refractory metal compound not readilywettable by binder metal is coated with a thin layer of wettable carbidefirmly 'bonded to the core. The thus coated core is now easily bonded toand wetted by metals of the iron group. It is important that this layernot react adversely over a large temperature range with the material inthe core or with the actual binder phase, which usually consists ofcobalt metal.

In carrying out the present method, a metal halide in the flu d or geous state is bro ght in contact with the 4 i hard material powder coresunder conditions which provide the formation of the desirable metalcarbide layer on the cores. Where the cores are comprised of refractorymetal carbides, the contact with the metal halide can be made directlyso as to result in an exchange of reaction; for instance, TiC+WCl TiCl+WC. Where the cores are borides or nitrides, an atmosphere comprisinghydrocarbon and preferably hydrogen is used together with the metalhalide vapor. The thickness of the layer can be varied from some 0.01micron up to several microns according to the temperature employed, therelation of the halide to reducing agent present, for instancehydrocarbon (e.g. methane), the grain size of the core and the reactiontime.

According to the present method, it is possible to coat grains oftitanium carbide (TiC) having a diameter of about 1 micron with a0.01-0.1 micron thick layer of pure WC. All the suitable layer types,for instance WC, M0 0, Cr C Ta C Nb C and V C wherein y=1 or 2 and x21,can with advantage be obtained by using the corresponding metal halidetogether with either hydrogen and/or a suitable hydrocarbon. Where y=1,the x should be less than 1 and range preferably from about 0.75 to0.85. It is also possible to carry out the deposition of the layer onthe core in two steps, substantially pure metal being deposited in thefirst step and the metal then being thereafter carburized in a secondstep. Carburization is preferably carried out with gaseous hydrocarbons(e.g. methane) and the temperature may be in the neighborhood of about1000 C., although other carburizing agents can be employed. Thisstep-wise treatment is especially suitable when a substoichiometriccarbide layer is desired, that is, a carbide layer based on the formulaMeC where x is less than 1 and preferably ranges from about 0.75 toabout 0.85. The particular carbide layer desired can be determined byexperiment by the controlling amount of reactants employed.

The invention will now be described in its more detailed aspects asfollows. Very hard, normally non-wettable core grains of TiC are coatedwith a thin layer of carbide from Group Vb, e.g. NbC or TaC where x 1.0but optimally ranges from about 0.75 to 0.85. The coated grains are hardthroughout with H 3000 kg./mm. yet; the grains can easily be bonded byeither Co, Ni or Fe. The hard phase in the sintered product is presentas angular grains. The layer material for imparting improved wettabilitymay also be V 0, Nb C or Ta C.

In another embodiment, the very hard grain of TiC is coated with a layerof tungsten carbide. While the coated grain is also quite hard, it isless hard than TiC. The coated grain can easily be bonded to Co, Ni orFe binder metal. The hard phase in the product appears as slightlyrounded grains after sintering. The grains of TiC as core material maybe replaced with other hard metal compounds such as TiB WB', TiN. Wherethe layer consists of TaC it does not react adversely with the TiC core,and the diffusion is also moderate. The layer of TaC which is firmlybonded to the surface of TiC, is easily wetted by nickel binder metaland a fine grain alloy is obtained after sintering at 1450 C. Thesintered body obtained exhibits hardnesses (H 3 kgs.) of up to about1600 kgs./mm. when the binder metal consists of less than about 20% byweight. The bending strength (trans verse rupture) is above 250 kg/mm.and compares very favorably with conventional products, very elastic butless hard metal of WC-Co, showing a bending strength of about 250kg./mm. and a hardness (H 3 kgs.) of about 1300 kg./mm.

It has been noted that a deposited layer of WC does not react adverselywith a core grain of TiC and that the diffusion is relativelyinconsequential. Therefore, a deposited layer of WC remains quite thinand wellas about 1200 C. Such temperatures are known to be obtained atthe tip of cuttin tools in machining operations. This has been confirmedby microprobe analyses. The layer of WC is firmly bonded to the surfaceof the titanium carbide grain and is detectable as a well defined layerby the microprobe. The deposited layer of WC is easily wetted and formsa firm bond with the elastic or ductile cobalt phase when sintered atabout 1400 C. The resulting hard metal alloy has a high hardness, highbending strength (transverse rupture) and exhibits high resistance towear. In a hard metal body comprising 60 weight percent of TiC, weightpercent of WC and 30 weight percent of Co, a hardness (H S kg.) of 1600kg./mm. and a bending strength of more than 300 kg./ mm. are obtained.The titanium carbide is present as somewhat rounded fine grains. Bycomparison, the system of 87 weight percent of WC and 13 weight percentof Co exhibits a hardness (H 3 kg.) of about 1200 kg./mm.? and a bendingstrength of about 250 kg./mm. The average grain size of the carbide inthe two systems is about 3 microns. Uncoated TiC generally grows to acoarse grain, e.g. l5 microns, compared to sintered coated TiC.

As illustrative of additional embodiments of the invention, thefollowing examples are given.

EXAMPLE 1 A kg. of TiC powder, averaging about 3 microns in size, wasreacted with gaesous WCl at 950 C. for about 1 mour. An analysis showedthat the coated TiC powder contained a little more than 9% W. The coatedpowder was treated in pure hydrogen at 1350 C. for about /2 hour, afterwhich the powder was analyzed again. Only T iC and WC phases could bedetected by X-ray measurements using the Guinier-H'zigg method. Thecoated TiC powder was then mixed for 50 hours in a ball mill togetherwith 30% Co binder metal and 2% wax. After pressing a compact at about 6to 10 t.s.i., the presintering of the compact was carried out at about900 C. for about one hour in hydrogen and the final sinterin at about1420 C. for about 1 /2 hours under vacuum. The sintered bodies weretested as regards transverse rupture strength by applying a load to abar 6 mm. high, 4 mm. wide and mm. long to failure and the porositytested according to ASTM B 276-54, the hardness according to Vickersusing 3 kgs. load, and a microstructure obtained at 150 timesmagnification. All tests were carried out-in comparison with a standardbody of TiC-30% Co, which standard body was also treated analogouslywith the schedule recited above. The geometry of the WC layer wasstudied indetail by a microprobe and the layer was shown to be very welldefined and narrow. The results obtained are given as follows:

TABLE 1 Uncoated TiC Coated TiC Composition TiC-30% Co Tig(10% WC)-30%Transverse rupture 129 kgJmm. 284=|=19 kg./mm.

strength. Porosity Large pores B4 A2. Hardness Dtfiicult to measure...1,598 kgJmm. Microstructure at 1,500 Coarse TiC-grains Fine granular TiCtimes magnification. generally over 15 generally less than 4 microns.

microns.

sintering and appears quite difiuse as compared to Example 1 where theWC layer is formed directly'on the grain before sintering. Only amarginal improvement in properties was obtained in comparison with theTiC-30% Co system. The results obtained are as follows:

Table 2 5 Transverse rupture strength 160:21 kg./mm.

Porosity A3. Hardness 1315 kg./mm. Microstructure 1500 timesmagnification TiC, 6-7 microns.

EXAMPLE 3 A kg. of WB powder, averaging 3 microns in size, was reactedwith gaseous TaCl and CH at 950 C. for about 1 hour. Analysis showedthat the WB powder contained about 6% Ta. The powder was treated atabout 1350 C. in hydrogen for /2 hour after which WB and TaC phases weredetected using X-ray and microprobe analyses. The coated WB powder wasmixed for 50 hours in a ball mill together with 10% Ni and 2% wax. Acompact was produced as in Example 1 and the compact then presintered at900 C. in hydrogen for about 1 hour and finally sintered at about 1450C. for about 1 /2 hours in vacuum. The sintered body was tested incomparison to a standard body containing WB-10% Ni as-recited in Example 1. The results obtained are as follows:

The TaC coating on the WB core corresponded to the formula MeX with xless than 1 and ranging from about 0.75 to 0.85.

While the invention has been described with regard to' producingsintered hard metal compositions from normally difiicult-to-wetrefractory metal compound grains or particles, it will be appreciatedthat the advantages of the invention can also be utilized in producinghard metal compositions in which part of the coated grains may bereplaced with refractory metal carbide grains or particles. which arewettable by iron group metals and alloys thereof. Examples of wettablecarbides which can be mixed with the coated hard metal grains inproducing sintered hard metal compositions are WC, MOgC, Cr C mixedcrystals of at least two carbides thereof, as well as mixed crystals(e.g. solid solution) of the system WC-TiC, among others. The amount ofwettable refractory metal carbide that can be mixed with and replacesome of the coated hard metal grains may range optionally from about 0to 10 times the amount ofcoated grains present.

Broadly speaking, the total amount of hard metal particles in thesintered hard metal composition may range from about 30% to 96% byweight, with substantially the balance an iron group binder metalranging from about 70% to 4% by weight. Advantageously, the sinteredcomposition may range in its more preferred aspects from about 5 0% to95% by weight of hard metal particles, with substantially the balancebinder metal ranging from about 50% t0 5% by weight. The binder metal isselected from the group consisting of iron, nickel and cobalt, andironbase, nickel-base and cobalt-base alloys. An example of an iron-basealloy is a steel comprising 5% Cr, 5% Mo, 0.5%

C and the balance'essentially iron. A nickel-base alloy is' TABLE 4Composition by weight of coated Composition by weight N 0. hard metalparticles of binder metal 1 30% (TiC coated with WC) 70% cobalt. 2 40%(ZrC coated with V 10) 60% nickel. 3 50% (NbC coated with 'IaCn-s) 50%iron alloy. 4 60% (WE coated with M020) 40% (80% Ni-20% Mo). 5 80% (TlBgcoated with @1302) 20% cobalt alloy. 6 90% (CrN coated with Nbcu's) 10%cobalt.

X 5% Cr, 5% M0, 0.5% C and Fe balance. 9 20% Cr, 5% W, 1% C and Cobalance.

As illustrative of cemented hard metal compositions in which wettablerefractory metal carbide particles are employed mixed with the coatedhard metal particles, the following examples are given in Table 5 below:

TABLE 5 Ratio by weight: of Composition by weight of coated uucoatedComposition and uncoated hard metal to coated by weight of No. particlesparticles binder metal 7--. 40% (30% T10 coated with W0, 10% 0.33 60%nickel CIzCz grains). alloy. 8 50% (25% WE coated with TaCo-a, 1.0 50%cobalt.

25% W grains). 0"..- 60% (25% TiN coated with NbCQ- 1.4 40% iron.

35% M020 grains). 10--.- 75% (25% CrN coated with MozC, 2.0 25% cobalt.

50% WC-TiC mixed crystals). 11-..- 90% (60% TiC coated with VCo-a, 0.6710% cobalt.

40% WC grains). 12..-- 90% (10% TiO coated with W0, 10% 3.5 Do.

TaC goated with W0, 70% W0 grains 13.-.- 94% (8% TiC coated with TaCo-s,10.75 6% cobalt.

86% W0 grains).

1 Cr, 5% M0, 0.1% O and Ni balance.

The foregoing examples of Table 5 illustrate how part of the coatedgrains can be replaced by wettable refractory metal carbides, such asgrains of Cr C WC, M0 0, WC-TiC (mixed crystals), etc. As statedhereinbefore, part of the coated grains can be replaced optionally bywettable refractory carbide grains in amounts ranging from about 0 totimes the amount of coated grains present, the total hard metalparticles present falling within the range of about 30% to 96% by weightor, more advantageously, from about 50% to 95% by weight.

As has been stated hereinbefore, the wettable refractory metal carbidecoating is produced from a halide of the refractory metal, therefractory metal being deposited from the halide in the fluid stateheated to the appropriate temperature. The fluid halide may be either inthe liquid or gaseous state (vapor), the gaseous state beingparticularly preferred. Following the production of the metal coating,the coating is thereafter carburized using solid or gaseous carburizingagents; or the carbide coating may be produced in situ using a mixtureof the halide vapor and a hydrocarbon gas, with or without hydrogenpresent.

Sintered hard metal compositions produced in accordance with theinvention, depending upon the amount of binder metal present, may beemployed in the production of cutting ools, tools in wh ch h gh hardnessand ear resistance are prime requisites, such as earth drilling tools,rests for centerless grinders, liners for brick mold, facings forhammers in hammermills, balls and seats for check valves, sandblastnozzles, ring and plug gages, gage blocks, wear pads for machinery, andthe like.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to be withthe purview and scope of the invention and the appended claims.

What is claimed is:

1. An improved sintered hard metal composition comprised essentially ofhard metal particles of refractory carbide-coated high temperaturerefractory metal compound grains dispersed through a binder matrix metalselected from the group consisting of Fe, Ni, Co and Febase, Ni-base andCo-base alloys, said coated refractory metal compound grains having acore selected from the group consisting of: (l) carbides of Ti, Zr, Hf,V, Nb and Ta, and (2) nitrides and borides of Ti, Zr, Hf, V, Nb, Ta, Cr,Mo and W, the refractory carbide coating on said core being selectedfrom the group consisting of carbides of V, Nb, Ta, Cr, Mo and W, thecarbides of V, Nb and Ta of said refractory carbide coating having theformula selected from the group consisting of Me C and MeC the compoundMeC having a cubic structure in which x is less than 1.

2. The sintered hard metal composition of claim 1 wherein x of theformula MeC ranges from about 0.75 to 0.85.

3. The sintered hard metal composition of claim 1, wherein the amount ofhard metal particles by weight ranges from about 30% to 96% and thebinder metal from about 70% to 4% by weight, and wherein the hard metalparticles may include optionally refractory metal carbide grainswettable by said binder metal to replace part of the coated grains, thetotal hard metal particles present falling within the range of about 30%to 96% by weight.

4. The sintered hard metal composition of claim 3, wherein the amount ofthe wettable refractory metal carbide ranges up to about 10 times theamount of coated grains present.

5. The sintered hard metal composition of claim 3, wherein the wettablerefractory metal carbide is selected from the group consisting of WC, MoC, Cr C mixed crystals of at least two of said carbides and mixedcrystals of the system WC-TiC.

6. An improved sintered hard metal composition comprised essentially ofhard metal particles of refractory carbide-coated refractory compoundgrains dispersed through a matrix metal selected from the groupconsisting of Fe, Ni, Co and Fe-base, Ni-base, and Co-base alloys, saidcoated refractory compound grains having a core selected from the groupconsisting of carbides of Ti, Zr, Hf, V, Nb and Ta, the refractorycarbide coating on said core being selected from the group consisting ofcarbides of V, Nb, Ta, Cr, Mo and W, the carbides of V, Nb and Ta orsaid refractory carbide coating having the formula selected from thegroup consisting of Me C and MeC the compound MeC having a cubicstructure in which .1: is less than 1 and ranges from about 0.75 toabout 0.85.

7. The sintered hard metal composition of claim 6, wherein the amount ofhard metal particles by weight ranges from about 30% to 96% and thebinder metal from about 70% to 4% by weight, and wherein the hard metalparticles may include optionally refractory metal carbide grainswettable by said binder metal to replace part of the coated grains, thetotal hard metal particles present falling within the range of about 30%to 96% by weight.

8. The sintered hard metal composition of claim 7, wherein the amount ofwettable refractory metal carbide ranges-up to about 10 times the amountof coated grains present. l

9. The sintered hard metal composition of claim 8, wherein the wettablerefractory metal carbide is selected from the group consisting of WC, MoC, Cr C mixed crystals of at least two of said carbides and mixedcrystals of the system WC-TiC.

10. An improved. sintered hard metal composition comprised essentiallyof hard metal particles of refractory carbide-coated high temperaturerefractory metal compound grains dispersed through a binder matrix metalselected from the group consisting of Fe, Ni, Co and Febase, Ni-base andCo-base alloys, said coated refractory metal compound grains having acore selected from the group consisting of nitrides of Ti, Zr, Hf, V,Nb, Ta, Cr, M and W, the refractory carbide coating on said core beingselected from the group consisting of carbides of V, Nb, Ta, Cr, Mo andW, the carbides of V, Nb, and Ta of said refractory carbide coatinghaving the formula selected from the group consisting of Me C and MeCthe compound MeC having a cubic structure in which x is less than 1 andranges from about 0.75 to about 0.85.

11. The sintered hard metal composition of claim 10, wherein the amountof hard metal particles by weight ranges from about 30% to 96% and thebinder metal from about 70% to 4% by weight, and wherein the hard metalparticles may include optionally refractory metal carbide grainswettable by said binder metal to replace part of the coated grains, thetotal hard metal particles present falling within the range of about 30%to 96% by weight.

12. The sintered hard metal composition of claim 11, wherein the amountof wettable refractory metal carbide ranges up to about times the amountof coated grains present.

13. The sintered hard metal composition of claim 11, wherein thewettable refractory metal carbide is selected from the group consistingof WC, Mo C, Cr C mixed crystals of at least two of said carbides andmixed crystals of the system WC-TiC.

14. An improved sintered hard metal composition comprised essentially ofhard metal particles of refractory carbide-coated high temperaturerefractory metal compound grains dispersed through a binder matrix metalselected from the group consisting of Fe, Ni, Co and Febase, Ni-base andCo-base alloys, said coated refractory metal compound grains having acore selected from the group consisting of borides of Ti, Zr, Hf, V, Nb,Ta, Cr, Mo and W, the refractory carbide coating on said core beingselected from the group consisting of carbides of V, Nb, Ta, Cr, Mo andW, the carbides of V, Nb and Ta of said refractory carbide coatinghaving the formula selected from the group consisting of Me C and MeCthe compound MeC having a cubic structure in which x is less than 1 andranges from about 0.75 to about 0.85.

15. The sintered hard metal composition of claim 14, wherein the amountof hard metal particles by weight ranges from 30% to 96% and the bindermetal from about 70% to 4% by weight, and wherein the hard metalparticles may include optionally refractory metal carbide grainswettable by said binder metal to replace part of the coated grains, thetotal hard metal particles present falling within the range of about 30%to 96% by weight.

16. The sintered hard metal composition of claim 15, wherein the amountof wettable refractory metal carbide ranges up to about 10 times theamount of coated grains present.

17. The sintered hard metal composition of claim 15, wherein thewettable refractory metal carbide is selected from the group consistingof WC, Mo C, Cr C mixed crystals of at least two of said carbides andmixed crystals of the system WC-TiC.

18. A method of producing an improved sintered hard metal compositioncomprising hard metal particles of refractory metal compound grainsnormally diflicult to wet with a binder matrix metal selected from thegroup consisting of the iron group metals Fe, Ni, Co and Febase, Ni-baseand Co-base alloys which comprises, selecting a batch of refractorymetal compound core grains from the group consisting (l) carbides of Ti,Zr, Hf, V, Nb and Ta, and (2) nitrides and borides of Ti, Zr, Hf, V, Nb,Ta, Cr, Mo and W, applying to said core grains a firmly bonded coatingof at least one metal carbide selected from the group consisting of V,Nb, Ta, Cr, Mo and W, the carbides of V, Nb and Ta of said metal carbidecoating having the formula selected from the group consisting of Me Cand MeC the compound MeC having a cubic structure in which x is lessthan 1, forming a powder mixture of said coated grains and said bindermatrix metal, compressing said mixture into a compact, sintering saidcompact at an elevated temperature at which said binder melts, and thencooling the sintered compact, whereby a strongly bonded hard metalcomposition is obtained.

19. The method of claim 18, wherein x of the formula MeC ranges fromabout 0.75 to about 0.85.

20. The method of claim 18, wherein the amount of hard metal particlesby weight ranges from about 30% to 96% and the binder metal from about70% to 4% by weight, and wherein the coated hard metal particles mayinclude optionally in mixture therewith refractory metal carbide grainswettable by said binder metal to replace part of the coated grains, thetotal hard metal particles present falling within the range of about 30%to 96% by weight.

21. The method of claim 20, wherein the amount of wettable refractorymetal carbide ranges up to about 10 times the amount of coated grainspresent.

22. The method of claim 20, the wettable refractory metal selected fromthe group consisting of WC, Mo C, Cr C mixed crystals of at least two ofsaid carbides and mixed crystals of the system WC-TiC.

23. The method of claim 18, wherein said metal carbide coating isproduced on the core grains by contacting the core grains with a halideof at least one of said metals V, Nb, Ta, Cr, Mo and W at an elevatedtemperature with the halide in the fluid state.

24. The method of claim 23, wherein the halide is a vapor and whereinthe treatment is carried out in an atmosphere of hydrogen.

25. The method of claim 23, wherein the core grains are coated byheating them to an elevated temperature in an atmosphere comprising avapor of said halide and a hydrocarbon gas.

26. The method of claim 23, wherein said metal carbide coating is formedby applying a coating of a metal from the group consisting of V, Nb, Ta,Cr, Mo and W by reduction of a halide gas of staid metal in anatmosphere containing hydrogen, followed by a second step of carburizingsaid metal coating.

27. Refractory carbide-coated refractory metal compound grains suitablefor use in the production of sintered hard metal compositions, thecoated grains having a core selected from the group consisting of: (1)carbides of Ti, Zr, Hf, V, Nb and Ta, and (2) nitrides and borides ofTi, Zr, Hf, V, Nb, Ta, Cr, Mo and W, the refractory carbide coating onsaid core being selected from the group consisting of carbides of V, Nb,Ta, Cr, Mo and W, the carbides of V, Nb and Ta of said refractorycarbide coating having the formula selected from the group consisting ofMe C and MeC the compound MeC having a cubic structure in which x isless than 1.

28. The coated grains of claim 27, wherein x is the formula MeC rangesfrom about 0.75 to 0.85.

29. The coated grains of claim 27, wherein the core of the grains isselected from the group consisting of carbides of Ti, Zr, Hf, V, Nb andTa.

30. The coated grains of claim 27, wherein the core of the grains isselected from the group consisting of nitrides of Ti, Zr, Hf, V, Nb, Ta,Cr, Mo and W.

l1 12 31. The coated grains of claim 27, wherein the core 5,489,5411/1970 Steinberg- 51--295 of the grains is selected from the groupconsisting of 3,663,191 5/1972 Kroder 51295 borides of Ti, Zr, Hf, V,Nb, Ta, Cr, Mo and W. V

CA'RL D. QUA-RFORTH, Primary Examiner References Cited 5 B. HUNT,Assistant Examiner UNITED STATES PATENTS Y 3,520,667 7/1970 Taylor51-308 1 3,264,102 8/1966 Scrugg 75 212 29182-7, 182.8; 75-05 BC,- 201,203, 204, 205, 212;

2,1 19,487 5/1938 Padowiz 75-212 10643, 47 R; 117-100 B, 106 C

